I can't offer any advice on the practical side of things (pregnant with my first) but you said in a previous post that your family were making unhelpful comments about her looking happier after her FF. Its worth pointing out if they say this kind of thing again and you don't strangle them immediately that she is looking happier because she is full and that if you could fill her up on breast milk at every feed then you would and she'd be happy with that. There is nothing wrong with the quality of your milk.
After the advice to wean a baby under 8lbs, I'd be asking to see a different health visitor in future and putting in a complaint. I'm not saying that a health visitor shouldn't be concerned about the weight loss, of course they should, but it is ridiculous to suggest that solids would be better than breastmilk and formula.
The fact that she doesn't seem to be gaining on formula top ups (unless its just too early to tell, I'm not an expert) suggests that it may never have been a problem with your breastmilk/let down/breasts. She may be ill. I think you need to see a doctor about this, not a health visitor who is obviously clutching at straws if she has gone from 'Breastfeeding is the problem, Formula is the answer' to 'Ah well, Milk of any kind is the problem, Solids are the answer' in such a short period of time.
I'm not saying that top-ups aren't part of the solution for you and dd, a doctor may even agree that they'd rather see how much she's eating (so ebm and ff rather than breastfeeding) but there could be a medical reason for her not putting on weight. If a doctor can't find anything else wrong, then I think it would be worth having the TT sorted. Okay, its not a severe one so may not help, but as she has feeding difficulties with both breast and bottle, there really ought to be no harm in trying it.
If you are worried about her going 'off' the breast whilst doing topups even if they are necessary to monitor her feeding, you could get an at breast supplimentor as mentioned above.
Try to get some more supportive help than that HV and find a way of relaxing, this stress on you both probably isn't helping.